Abstract: The differential equation of the curved surface of down string-nets of bi-directional beam-string structures is deduced by the relationship of balanced loads and by the form of string-nets and the tensions of the strings. The FEM and corresponding program to calculate the curved string-net surface is provided according to the equilibrium condition of cable-nets. The FEM process to calculate the net surface is introduced by 2 examples. The method is of definite physical meanings and easy to carry out with accurate results.